A veteran Chinese activist who was involved in the 1989 Tiananmen protests has been jailed for 10 years.

Chen Xi was convicted of subverting state power after a trial lasting a few hours. He had published essays online criticising the Communist Party.

The jailing comes days after another activist, Chen Wei, was imprisoned for the same offence.

Rights groups expressed outrage and accused Beijing of using the Christmas period as cover for a crackdown.

"It does work really well because there's no diplomatic activity around Christmas," said Nicholas Bequelin of Human Rights Watch.

"By the time the diplomats get back to their desks, the sequence of events has moved on."

Chen Xi's wife, Zhang Qunxuan, told reporters that her husband was innocent, but would not launch a "futile" appeal against his conviction.

"Chen Xi told the court it did not take into consideration the things he has written as a whole, and has interpreted his words out of context. But they have power and they don't listen," she said.

He had posted 36 essays online, and also hosted a human rights forum in Guiyang, south-west China.

Chen Xi has been jailed several times since being involved in the 1989 protests.

Analysts say Beijing has a long-standing policy of punishing veteran activists who refuse to stop criticising the government.

Another activist, Chen Wei, was jailed last week for nine years for criticising the party.

He had argued that he was exercising the right of freedom of expression guaranteed by China's constitution.

UN human rights chief Navi Pillay called his sentence "extremely harsh" and said it "indicates a further tightening of the severe restrictions on the scope of freedom of expression in China that has been seen over the last two years".

"I call upon Chinese authorities to release any person detained for peacefully exercising his or her right to freedom of expression," she said in a statement released on Monday.

Ms Pillay also criticised the decision earlier this month to send lawyer Gao Zhisheng back to jail.

"[These] are the latest examples of an escalating clampdown on the activities of human rights defenders in China," she said.